Conductive contact for solar cell

ABSTRACT

Conductive contacts for solar cells and methods of forming conductive contacts for solar cells are described. For example, a solar cell includes a substrate. A conductive contact is disposed on the substrate. The conductive contact includes a layer composed of a first metal species having a plurality of pores. The conductive contact also includes a second metal species disposed in the plurality of pores. Portions of both the first and second metal species are in contact with the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/629,447, filed on Sep. 27, 2012, the entire contents of which arehereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the present invention are in the field of renewableenergy and, in particular, conductive contacts for solar cells andmethods of forming conductive contacts for solar cells.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present invention allow for increasedsolar cell manufacture efficiency by providing novel processes forfabricating solar cell structures. Some embodiments of the presentinvention allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional scanning electron microscope (X-SEM) imageof a portion of a contact for a solar cell, in accordance with anembodiment of the present invention.

FIG. 2 is a cross-sectional illustration of a portion of the contactstructure of FIG. 1, in accordance with an embodiment of the presentinvention.

FIG. 3 includes a plot of an EXD measurement of a portion of the metalstack of FIG. 1 (included as an X-SEM image in FIG. 3), in accordancewith an embodiment of the present invention.

FIGS. 4A-4C illustrate cross-sectional views of various processingoperations in a method of fabricating solar cells having conductivecontacts, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Conductive contacts for solar cells and methods of forming conductivecontacts for solar cells are described herein. In the followingdescription, numerous specific details are set forth, such as specificprocess flow operations, in order to provide a thorough understanding ofembodiments of the present invention. It will be apparent to one skilledin the art that embodiments of the present invention may be practicedwithout these specific details. In other instances, well-knownfabrication techniques, such as lithography and patterning techniques,are not described in detail in order to not unnecessarily obscureembodiments of the present invention. Furthermore, it is to beunderstood that the various embodiments shown in the figures areillustrative representations and are not necessarily drawn to scale.

Disclosed herein are solar cells having conductive contacts. In anembodiment, a solar cell includes a substrate. A conductive contact isdisposed on the substrate. The conductive contact includes a layercomposed of a first metal species having a plurality of pores. Theconductive contact also includes a second metal species disposed in theplurality of pores. Portions of both the first and second metal speciesare in contact with the substrate. In another embodiment, a solar cellincludes a silicon substrate. A conductive contact is disposed on thesilicon substrate. The conductive contact includes a first conductivelayer composed of aluminum (Al) containing particles having a pluralityof pores. The first conductive layer also includes nickel (Ni) disposedin the plurality of pores. Portions of both the Al containing particlesand the Ni are in contact with the silicon substrate. In anotherembodiment, a back-contact solar cell includes a bulk N-type siliconsubstrate. A conductive contact is disposed on a surface of the bulkN-type silicon substrate opposing a light receiving surface of the bulkN-type silicon substrate. The conductive contact includes a firstconductive layer composed of aluminum/silicon (Al/Si) alloy particleshaving a plurality of pores. Nickel (Ni) is disposed in the plurality ofpores. Portions of both the aluminum/silicon (Al/Si) alloy particles andthe Ni are in contact with the bulk N-type silicon substrate. A portionof the Ni forms a silicide with a portion of the bulk N-type siliconsubstrate. A second conductive layer composed substantially of Ni, butnot Al or Cu, is disposed on the first conductive layer. A thirdconductive layer composed substantially of copper (Cu) is disposed onthe second conductive layer.

Also disclosed herein are methods of fabricating solar cells havingconductive contacts. In an embodiment, a method of fabricating aback-contact solar cell includes forming a conductive contact on asurface of a bulk N-type silicon substrate opposing a light receivingsurface of the bulk N-type silicon substrate. The forming includesforming a first conductive layer composed of aluminum/silicon (Al/Si)alloy particles having a plurality of pores. Nickel (Ni) is disposed inthe plurality of pores. Portions of both the aluminum/silicon (Al/Si)alloy particles and the Ni are in contact with the bulk N-type siliconsubstrate. The method also includes forming, on the first conductivelayer, a second conductive layer composed substantially of Ni, but notAl or Cu. The method also includes forming, on the second conductivelayer, a third conductive layer composed substantially of copper (Cu).

One or more embodiments described herein are directed to approaches to,and structures resulting from, reducing the contact resistance ofprinted Al seed formed on a silicon substrate by incorporatingelectroless-plated Ni therein. For example, during fabrication of solarcontacts for solar cells it was realized that, e.g., by X-SEM imaging,following Ni deposition by electroless plating on top of printed Al, Niformed on the surface of Al particles. Such formation was accompanied bya filling of voids or pores present in an Al paste. The resultingcontacts suffered from less contact resistance as compared withconventional contacts, e.g., Al-only contacts. The results can beparticularly useful for the fabrication of back-contact solar cellswhere thermal budget and restricted silicon consumption can hinder goodmetal to silicon contact formation. However, it is to be understood thatembodiments described herein may be useful for both back-contact solarcells and front-contact solar cells.

Further to the above described issues, the contact resistance of printedcontacts tends to be lower than contacts formed by vacuum depositedmetal. One possibility for the reduced resistance is that the porosityof a seed paste can limit the actual contact areas between the metal andsilicon. There are a couple of conventional approaches normally used toreduce the contact resistance of a solar cell. The first involves tuningthe formulation design of the seed layer of the contacts. Differentadditives (e.g., binders/frits) can be added to improve the adhesionbetween the paste and the silicon substrate. However, there can be apractical limit of how much metal is in contact with the silicon afterfiring (e.g., annealing) the final contact metal stack. The secondconventional approach involves tuning the firing profile. Similar to thefirst approach, it can lower the contact resistance to a certain degree,but the porosity of the paste can still pose a limiting factor forattempts to reducing contact resistance.

More specifically, one or more embodiments are directed to contactformation starting with an Al paste seed layer. Annealing is performedafter seed printing to form contact between Al from the past and anunderlying silicon substrate. Then Ni is deposited by electrolessplating on top of Al paste. Since the paste has a porous structure, theNi forms not only above, but also on the outside of the Al particles,and fills up at least a portion of the empty space. The Ni may be gradedin that more Ni may form on upper portions of the Al (away from the Si).Nonetheless, the Ni on the outside of the Al particles can be utilizedto reduce the contact resistance of a contact ultimately formed therefrom. In particular, if the thickness of the Al paste is generallyreduced, more Ni can accumulate at the Al to silicon interface. Whenannealing is performed after Ni electroless plating, instead of afterseed printing, a NiSi contact can form at the Ni—Si interface.Furthermore, an Al—Si contact can form at the Al—Si interface by havingthe Ni present in voids or pores of the Al particles. Compared toconventional approaches, the contacts formed can have a greater surfacearea of actual metal to silicon contact within a given region of thecontact structure formation. As a result, the contact resistance can belowered relative to conventional contacts.

As an example of the concepts described above, FIG. 1 is across-sectional scanning electron microscope (X-SEM) image 100 of aportion of a contact for a solar cell, in accordance with an embodimentof the present invention. FIG. 2 is a cross-sectional illustration of aportion 200 of the contact structure of FIG. 1, in accordance with anembodiment of the present invention.

Referring to FIG. 1 a metal stack 104 of a contact structure for asubstrate 102 of a solar cell includes three discernible metal layerportions 106, 108 and 110. The first metal layer portion 106 is a seedlayer composed of a seed layer of porous Al particles. The Al particleshave Ni formed in the spaces and voids thereof, e.g., from Ni plating ontop of the Al seed layer. The second metal layer portion 108 is composedof Ni formed above, but not within, the Al particles from the same orsimilar Ni deposition process. As highlighted in FIG. 1, a higherconcentration of Ni is present at the upper surface of the first metallayer portion 106 than at the bottom of that layer. The third metallayer portion 110 is composed of copper formed above the second metallayer portion 108.

Referring to FIG. 2, although the concentration of Ni can be higher atthe upper surface of the first metal layer portion 106 than at thebottom of that layer, Ni can still come in contact with the siliconsubstrate surface nonetheless. Accordingly, at the interface ofsubstrate 102 and the first metal layer portion 106, both Al/Si and NiSicontacts can be formed, as depicted in FIG. 2. Furthermore, in anembodiment, since a greater concentration of Ni can form on the top(away from Si), if the thickness of the Al paste is reduced, the amountof Ni ultimately present at the Si interface can be tailored. Also, inan embodiment, when annealing is performed, a highly conductive nickelsilicide (NiSi) material can be formed underneath the Ni—Si interface.

Accordingly, referring to both FIGS. 1 and 2, in an embodiment, a solarcell includes a substrate 102. A conductive contact 104 is disposed onthe substrate 102. The conductive contact 104 includes a layer 106composed of a first metal species having a plurality of pores. Theconductive contact 104 also includes a second metal species disposed inthe plurality of pores of the layer 106. Portions of both the first andsecond metal species are in contact with the substrate 102.

In an embodiment, the first metal species is aluminum (Al) and thesecond metal species is nickel (Ni). In one such embodiment, thealuminum is included as a plurality of aluminum/silicon (Al/Si) alloyparticles. In a specific such embodiment, a majority of the Al/Si alloyparticles has a diameter approximately in the range of 1-5 microns, anda majority of the pores has a diameter approximately in the range of0.1-1 microns. In an embodiment, the conductive contact 104 is aback-contact for the solar cell. However, in an alternative embodiment,the contact 104 is a front contact for the solar cell.

In an embodiment, a concentration of the second metal species is gradedhigher in concentration distal from the substrate 102 and lower inconcentration proximate to the substrate 102. In an alternativeembodiment, however, a concentration of the second metal species ishomogeneous throughout the plurality of pores of the first metalspecies. In an embodiment, the substrate 102 is a bulk siliconsubstrate, and a portion of the second metal species forms a silicidewith a portion of the bulk silicon substrate. In an embodiment, thesolar cell further includes a layer 110 composed substantially of athird metal species and disposed above the layer 106 including the firstmetal species and the second metal species. In one such embodiment, thethird metal species is copper (Cu). In another such embodiment, a thirdlayer 108 composed substantially of the second metal species, but notthe first or second metal species, is disposed between the layer 110 andthe layer 106.

Referring again to both FIGS. 1 and 2, in another embodiment, a solarcell includes a silicon substrate 102. A conductive contact 104 isdisposed on the silicon substrate 102. The conductive contact includes afirst conductive layer 106 composed of aluminum (Al) containingparticles having a plurality of pores. The first conductive layer 106also includes nickel (Ni) disposed in the plurality of pores. Portionsof both the Al containing particles and the Ni are in contact with thesilicon substrate 102.

In an embodiment, a concentration of the Ni is graded higher inconcentration distal from the silicon substrate 102 and lower inconcentration proximate to the silicon substrate 102. In an alternativeembodiment, however, a concentration of the Ni is homogeneous throughoutthe plurality of pores of the Al containing particles. In an embodiment,the Al containing particles are aluminum/silicon (Al/Si) alloyparticles. In an embodiment, a portion of the Ni forms a silicide with aportion of the silicon substrate 102. In an embodiment, the solar cellfurther includes another conductive layer 110 composed substantially ofcopper (Cu) and disposed above the first conductive layer 106. Anotherconductive layer 108 composed substantially of Ni, but not Al or Cu, isdisposed between the conductive layers 106 and 110.

Referring again to both FIGS. 1 and 2, in another embodiment, aback-contact solar cell includes a bulk N-type silicon substrate 102. Aconductive contact 104 is disposed on a surface of the bulk N-typesilicon substrate 102 opposing a light receiving surface 103 of the bulkN-type silicon substrate 102. The conductive contact 104 includes afirst conductive layer 106 composed of aluminum/silicon (Al/Si) alloyparticles having a plurality of pores. Nickel (Ni) is disposed in theplurality of pores. Portions of both the aluminum/silicon (Al/Si) alloyparticles and the Ni are in contact with the bulk N-type siliconsubstrate 102. A portion of the Ni forms a silicide with a portion ofthe bulk N-type silicon substrate 102. A second conductive layer 108composed substantially of Ni, but not Al or Cu, is disposed on the firstconductive layer 106. A third conductive layer 110 composedsubstantially of copper (Cu) is disposed on the second conductive layer108. In one embodiment, a concentration of the Ni is graded higher inconcentration distal from the bulk N-type silicon substrate 102 andlower in concentration proximate to the bulk N-type silicon substrate102. In an alternative embodiment, a concentration of the Ni ishomogeneous throughout the plurality of pores of the aluminum/silicon(Al/Si) alloy particles.

As an example of experimental verification that Ni can be used to fillvoids in Al particles from an Al paste, FIG. 3 includes a plot 300 of anEXD measurement of a portion of the metal stack of FIG. 1 (included asX-SEM image 302), in accordance with an embodiment of the presentinvention. Referring to plot 300 and image 302, the experimental dataverifies that the metal species formed in an around the Al particles isNi.

Although certain materials are described specifically above, somematerials may be readily substituted with others with other suchembodiments remaining within the spirit and scope of embodiments of thepresent invention. For example, in an embodiment, a different materialsubstrate, such as a group III-V material substrate, can be used insteadof a silicon substrate. In another embodiment, silver (Ag) particles orthe like can be used in a seed paste instead of, or in addition to, Alparticles. In another embodiment, plated or like-deposited cobalt (Co)or tungsten (W) can be used instead of or in addition to the plated Nidescribed above.

Furthermore, the formed contacts need not be formed directly on a bulksubstrate. For example, in one embodiment, conductive contacts such asthose described above are formed on semiconducting regions formed above(e.g., on a back side of) as bulk substrate. As an example, FIGS. 4A-4Cillustrate cross-sectional views of various processing operations in amethod of fabricating solar cells having conductive contacts, inaccordance with an embodiment of the present invention.

Referring to FIG. 4A, a method of forming contacts for a back-contactsolar cell includes forming a thin dielectric layer 402 on a substrate400.

In an embodiment, the thin dielectric layer 402 is composed of silicondioxide and has a thickness approximately in the range of 5-50Angstroms. In one embodiment, the thin dielectric layer 402 performs asa tunneling oxide layer. In an embodiment, substrate 400 is a bulksingle-crystal substrate, such as an n-type doped single crystallinesilicon substrate. However, in an alternative embodiment, substrate 400includes a polycrystalline silicon layer disposed on a global solar cellsubstrate.

Referring again to FIG. 4A, trenches 416 are formed between n-type dopedpolysilicon regions 420 and p-type doped polysilicon regions 422.Portions of the trenches 416 can be texturized to have textured features418, as is also depicted in FIG. 4A.

Referring again to FIG. 4A, a dielectric layer 424 is formed above theplurality of n-type doped polysilicon regions 420, the plurality ofp-type doped polysilicon regions 422, and the portions of substrate 400exposed by trenches 416. In one embodiment, a lower surface of thedielectric layer 424 is formed conformal with the plurality of n-typedoped polysilicon regions 420, the plurality of p-type doped polysiliconregions 422, and the exposed portions of substrate 400, while an uppersurface of dielectric layer 424 is substantially flat, as depicted inFIG. 4A. In a specific embodiment, the dielectric layer 424 is ananti-reflective coating (ARC) layer.

Referring to FIG. 4B, a plurality of contact openings 426 are formed inthe dielectric layer 424. The plurality of contact openings 426 provideexposure to the plurality of n-type doped polysilicon regions 420 and tothe plurality of p-type doped polysilicon regions 422. In oneembodiment, the plurality of contact openings 426 is formed by laserablation. In one embodiment, the contact openings 426 to the n-typedoped polysilicon regions 420 have substantially the same height as thecontact openings to the p-type doped polysilicon regions 422, asdepicted in FIG. 4B.

Referring to FIG. 4C, the method of forming contacts for theback-contact solar cell further includes forming conductive contacts 428in the plurality of contact openings 426 and coupled to the plurality ofn-type doped polysilicon regions 420 and to the plurality of p-typedoped polysilicon regions 422. In an embodiment, the conductive contacts428 are composed of metal and are formed by a deposition (described ingreater detail below), lithographic, and etch approach.

Thus, in an embodiment, conductive contacts 428 are formed on or above asurface of a bulk N-type silicon substrate 400 opposing a lightreceiving surface 401 of the bulk N-type silicon substrate 400. In aspecific embodiment, the conductive contacts are formed on regions(422/42) above the surface of the substrate 400, as depicted in FIG. 4C.The forming can include forming a first conductive layer composed ofaluminum/silicon (Al/Si) alloy particles having a plurality of pores,and nickel (Ni) disposed in the plurality of pores. Portions of both thealuminum/silicon (Al/Si) alloy particles and the Ni are in contact withthe regions 420/422 above the bulk N-type silicon substrate 400 (or inother embodiments, in direct contact with the bulk N-type siliconsubstrate 400). A second conductive layer is formed on the firstconductive layer is composed substantially of Ni, but not Al or Cu. Athird conductive layer is then formed on the second conductive layer iscomposed substantially of copper (Cu).

In an embodiment, forming the first conductive layer includes printing apaste on the bulk N-type silicon substrate. The paste is composed of asolvent and the aluminum/silicon (Al/Si) alloy particles. The printingincludes using a technique such as, but not limited to, screen printingor ink jet printing.

In an embodiment, forming the first conductive layer includes depositingthe Ni by an electroless deposition technique. In an embodiment, formingthe first conductive layer includes heating the aluminum/silicon (Al/Si)alloy particles, but not the Ni, to a temperature approximately in therange of 500-600 degrees Celsius. In an embodiment, forming the firstconductive layer includes heating the aluminum/silicon (Al/Si) alloyparticles and the Ni to a temperature approximately in the range of500-600 degrees Celsius. In an embodiment, the forming of the Ni of thefirst conductive layer and the forming of the second conductive layerare performed in the same process operation. In an embodiment, formingthe third conductive layer includes depositing the Cu by anelectro-plating deposition technique.

Thus, conductive contacts for solar cells and methods of formingconductive contacts for solar cells have been disclosed. In accordancewith an embodiment of the present invention, a solar cell includes asubstrate. A conductive contact is disposed on the substrate. Theconductive contact includes a layer composed of a first metal specieshaving a plurality of pores. The conductive contact also includes asecond metal species disposed in the plurality of pores. Portions ofboth the first and second metal species are in contact with thesubstrate. In one embodiment, the first metal species is aluminum (Al)and the second metal species is nickel (Ni).

What is claimed is:
 1. A solar cell, comprising: a substrate; and aconductive contact disposed on the substrate, the conductive contactcomprising a layer comprising a first metal species having a pluralityof pores, and a different second metal species disposed in the pluralityof pores, wherein portions of both the first and second metal speciesare in contact with the substrate, wherein a concentration of the secondmetal species is graded higher in concentration distal from thesubstrate and lower in concentration proximate to the substrate, andwherein a majority of the plurality of pores has a diameterapproximately in the range of 0.1-1 microns, wherein the first metalspecies in aluminum (Al), wherein the aluminum is included as aplurality of aluminum/silicon (Al/Si) alloy particles, and wherein amajority of the Al/Si alloy particles has a diameter approximately inthe range of 1-5 microns.
 2. The solar cell of claim 1, wherein thesubstrate is a bulk silicon substrate, and wherein a portion of thesecond metal species forms a silicide with a portion of the bulk siliconsubstrate.
 3. The solar cell of claim 1, further comprising: a secondlayer substantially comprising a third metal species disposed above thelayer comprising the first metal species and the second metal species.4. The solar cell of claim 3, wherein the third metal species is copper(Cu).
 5. The solar cell of claim 3, further comprising: a third layersubstantially comprising the second metal species, but not the first orsecond metal species, the third layer disposed between the second layerand the layer comprising the first metal species and the second metalspecies.
 6. The solar cell of claim 1, wherein the conductive contact isa back-contact.
 7. The solar cell of claim 1, wherein the second metalspecies is nickel (Ni).
 8. A back-contact solar cell, comprising: a bulkN-type silicon substrate; and a conductive contact disposed on a surfaceof the bulk N-type silicon substrate opposing a light receiving surfaceof the bulk N-type silicon substrate, the conductive contact comprising:a first conductive layer comprising aluminum/silicon (Al/Si) alloyparticles having a plurality of pores, and a different second metalspecies disposed in the plurality of pores, wherein portions of both thealuminum/silicon (Al/Si) alloy particles and the second metal speciesare in contact with the bulk N-type silicon substrate, wherein a portionof the second metal species forms a silicide with a portion of the bulkN-type silicon substrate, and wherein a majority of the plurality ofpores has a diameter approximately in the range of 0.1-1 microns; asecond conductive layer substantially comprising the second metalspecies, but not Al, the second conductive layer disposed on the firstconductive layer; and a third conductive layer substantially comprisinga different third metal species disposed on the second conductive layer,wherein a concentration of the second metal species is graded higher inconcentration distal from the bulk N-type silicon substrate and lower inconcentration proximate to the bulk N-type silicon substrate, wherein amajority of the Al/Si alloy particles has a diameter approximately inthe range of 1-5 microns.
 9. The back-contact solar cell of claim 8,wherein the second metal species is nickel (Ni).